Porous materials are defined as those that:

Porous materials are specifically characterized by their ability to allow liquids to be absorbed. This means that they have small openings or spaces (pores) that can take in liquid substances. This property is significant in various fields, including those related to health and safety, as porous materials can be more challenging to clean and disinfect compared to non-porous materials. Understanding the nature of porous materials is vital in esthetic practices because it impacts the choice of tools and products for skin care treatments, ensuring they maintain hygiene and safety standards.

The other options do not accurately reflect the definition of porous materials. For instance, materials that do not absorb liquids are considered non-porous, and while porous materials can be disinfected, they generally require more effort and care in the process. Resistant to stains also refers more to the durability of the surface rather than its porosity. Hence, the defining characteristic of porous materials is indeed their ability to absorb liquids.